1. Field of the Invention
The present invention relates to an infrared ray cut filter used in an imaging apparatus that includes solid imaging elements and the like and to a manufacturing method therefore, and relates more specifically to an reflective-type infrared ray cut filter that uses a multilayer optical membrane.
2. Description of the Related Art
While human vision lacks sufficient optical sensitivity to perceive infrared light, i.e., light having a wavelength above approximately 700 nanometers (nm), a CCD (charge coupled device) has a sensitivity that enables it to perceive infrared light up to a wavelength of about 1100 nm. As a result, an infrared ray cut filter is used in an imaging apparatus that employs solid imaging elements such as CCDs. An infrared ray cut filter has the characteristic of blocking infrared light while allowing visible light to pass through. Consequently, the imaging apparatus can obtain images with an optical sensitivity close to that of human beings while blocking infrared light from CCDs or other devices.
Infrared ray cut filters include absorption-type and reflection-type filters. In an absorption-type infrared ray cut filter, infrared-absorbing metal ions are contained in glass. An absorption-type filter exhibits gradually decreasing light permeability as the light wavelength increases from 550 nm to 750 nm. This light permeability characteristic permits natural color reproduction with a sensitivity characteristic close to that of human vision, but in order to obtain good light permeability, the glass must have a prescribed minimum thickness. As a result, it is difficult to make the imaging apparatus small in size.
In a reflection-type infrared ray cut filter, on the other hand, a multilayer optical membrane that reflects infrared light is formed on glass. Because this type of filter can be realized by applying a multilayer optical membrane coating over an optical member incorporated in the imaging apparatus, and also because the multilayer membrane is thin, it is easy to make the imaging apparatus compact. However, the light permeability of a typical multilayer optical membrane filter decreases sharply between the permeable light range and the non-permeable light range. As a result, it is difficult to achieve natural color reproduction that closely reflects the sensitivity characteristic of human vision.
A reflection-type infrared ray cut filter that eliminates the above-mentioned light permeability problem is known in the conventional art. By forming the multilayer optical membrane to include at least 34 layers each having a prescribed optical thickness, light permeability is made to decrease gradually as the wavelength increases from 550 nm to 750 nm, in the same fashion as an absorption-type infrared ray cut filter. This technology is described in, for example, Japanese Laid-Open Patent No. 2000-314808.
However, this type of reflective-type infrared ray cut filter must constitute a multilayer optical membrane composed of at least 34 thin membrane layers. Consequently, variations in optical thickness that occur when the various thin membrane layers are formed have a significant effect on the light permeability of the multilayer optical membrane as a whole, making it difficult to manufacture a product with stable light permeability. As a result, it is costly and time-consuming to manufacture this type of infrared ray cut filter.
The present invention was created in view of the foregoing, and an object thereof is to provide a reflection-type infrared ray cut filter and associated manufacturing method that can reduce the cost and time required for manufacture and increase manufacturing efficiency.